1. Field of the Invention
The present invention relates to multi-shaft auger systems and processes for mixing soil with a chemical hardener in situ to form soil-cement columns, walls, piles, grids and monolithic block of overlapping columns. More particularly, the present invention is directed to improvements in auger shafts which permit more efficient penetration and improved mixing of the chemical hardener with the soil which forms the soil-cement columns, walls, piles, grids, and monolithic block of columns.
2. The Relevant Technology
The term "chemical hardener" includes any chemicals and agents that can be added and mixed with soil to cause chemical reactions. Examples of chemicals and agents are: portland cement, lime, fly ash, kiln dust, cement-based hardeners, bitumen, resin, power plant residues, bentonite, salts, acids, sodium and calcium silicates, calcium aluminates, and sulfates. The chemical reactions include possolanic reaction (cementation), hydration, ion-exchange, polymerization, oxidation, and carbonation. The results of these chemical reactions include changes in the physical properties of soil such as strength and permeability and/or the change of chemical properties such as the reduction of the toxicity level in contaminated soil or sludge. The chemical hardener is added in a slurry form. Therefore, the term "slurry" as used herein is defined as including chemical hardener. A soil-cement column is one of the most common products of in situ mixing of soil and chemical hardener, so it is used as a generic term to describe the hardened product of in situ soil mixing. In some cases, non-hardening soil-chemical or soil agent mixtures are desirable and should be considered within the scope of this invention.
For a number of years, multi-shaft auger machines have been used to construct soil-cement columns in the ground without having to excavate and remove the soil. These columns are sometimes referred to as "soilcrete" columns, because the soil is mixed with a cement slurry. Upon hardening, the soil-cement columns possess some characteristics of lower strength concrete columns, but they are constructed without the expense and time-consuming process of removing and replacing the soil with concrete. Cement slurry has also been called cement grout or cement milk in some of the previous art.
Soil-cement columns have been arranged in a variety of patterns depending on the desired application. Soil-cement columns are used to improve the load bearing capacity of soft soils, such as sandy or soft clay soils. The columns are formed deep in the ground to help support surface construction on soft soils.
In other cases, the soil-cement columns have been overlapped to form boundary walls, excavation support walls, low to medium capacity soil-mixed caissons, and for the in situ fixation of contaminated soil or toxic wastes.
To produce soil-cement columns, a multi-shaft auger machine bores holes in the ground and simultaneously mixes the soil with a slurry or slurties of chemical hardener pumped from the surface through the auger shaft to the end of the auger. Multiple columns are prepared while the soil-cement mixture or soil-chemical mixture is still soft to form continuous walls of geometric patterns within the soil depending on the purpose of the soil-cement columns.
Because the soil is mixed in situ and because the soil-cement wall is formed in a single process, the construction period is shorter than for other construction methods. Obviously, the costs of forming soil-cement columns are less than traditional methods requiring excavation of the soil, constructing forms, and then pouring concrete into the forms in order to form the concrete pillars or walls. In addition, because the soil is not removed from the ground, there is comparatively less material produced in situ by such processes that must be disposed of during the course of construction.
Historically, a modified earth digging auger machine is used in the formation of in situ soil-cement columns. The boring and mixing operations are performed by multi-shaft drive units in order to make the process more efficient. The shafts typically have attached soil mixing paddles and auger blades which horizontally and vertically mix the soil with the hardening material, thereby producing a column having a homogeneous mixture of the soil and the chemical hardener.
As auger blades located at the lower end of each shaft of a multi-shaft drive unit penetrate the soil, the soil is broken loose and a chemical hardener slurry is injected into the soil through the ends of the hollow-stemmed augers which are attached to the shaft. The augers penetrate, break loose, and lift the soil to mixing paddles which further blend the slurry in the soil.
Due to the tremendous forces required to push the shaft downward and to turn the augers and the shaft, as well as the tendency of the multiple shafts to diverge due to varying soil conditions encountered by each shaft, support structures are provided which surround each shaft. The support structures allow the shafts to rotate, while simultaneously providing lateral support.
Support structures typically take the form of nonrotating bands surrounding each shaft and stabilizing bars securely attached to the nonrotating bands to maintain proper shaft spacing and alignment. These nonrotating bands and stabilizer bars can be constructed as separate elements or as a single unitary piece.
Typically, these bands and stabilizer bars are constructed to be removable for easy assembly and disassembly of the shafts of the multi-shaft drive unit and for easy repair and replacement of the auger blades and mixing paddles themselves. As these supporting structures serve to prevent diversion of the auger shafts out of a parallel configuration, the support structures must be located fairly near to the lower ends of the shafts where the impact of rocks and varying soil textures has the most effect on the shafts.
As the augers penetrate new soil, the soil is loosened and the loosened soil is forced past the nonrotating bands and stabilizer bar by the action of the rotating auger blades pushing soil up from below. As the newly loosened soil is urged past the support structures by the action of the auger blades at the lower end of the shaft, resistance is encountered in the vicinity of the nonrotating bands and stabilizer bar.
The passage of the soil around these support structures causes an increase in friction, a concomitant decrease in efficiency of mixing, a reduction in the rate of progress of the shaft through the soil, and a proportional increase in the amount of energy utilized to prepare a soil-cement column. After passing the nonrotating band and stabilizer bar, the soil is remixed with mixing paddles attached to the shaft above the nonrotating band and stabilizer bar.
While this auger system works well in sandy or porous soils, problems are encountered when auguring in clay or clay-like soils. When the auger blades located at the end of each shaft encounter clay soils, the augers fracture and separate the clays only to have the clays reaggregate before passing the nonrotating bands and stabilizer bar which support the shafts. This reaggregation or reagglomeration of clay soil can form a cylindrical plug. The natural tendency of clays to stick and coalesce is further exacerbated by the injection of the slurry. When combined with the slurry, the cylindrical clay plug greatly increases resistance to the passage of supporting structures such as the nonrotating band and stabilizer bar therethrough.
When sufficient pressure is exerted on the clay plug by the action of the augers on new soil being forced up from below, the clay plug is forced around the nonrotating bands and stabilizer bar into the area of the borehole above the supporting structures. Once the cylindrical plug reaches the mixing blades located above the supporting structures, the cylindrical plug must once again be fractured and separated and thoroughly mixed with the slurry. This reseparation further slows the progress of the augers by reducing the energy available for penetrating additional layers of soil.
The resistance caused by the reconsolidation of the clay soil below the supporting structures results in a reduced rate of progress by the auger machine through the soil. Further, there is significantly less homogenous mixing of the soil with the slurry. The cylindrical plug reformed beneath the support structures must undergo essentially the same fracturing process above the support structures as the process the soil was subjected to below the structures. The mixing blades and paddles located on the shafts above the supporting structures must not only mix the soil but also refracture and reseparate it.
As the soil must be separated twice, much more energy is utilized in the mixing process. This energy must be deducted from the total energy available for penetrating new soil layers. This reduction in available energy results in less efficient boring, both in rate of progress through the soil and in the thoroughness of mixing of the soil with the slurry.
From the foregoing, it will be appreciated that what is needed in the art is a multi-shaft auger system which increases the rate of progress of an auger machine through clay soils.
It would be another advancement in the art to provide a multi-shaft auger system for mixing soil with a chemical hardener in situ, which provides for a more homogenous mixture of a chemical hardener slurry and a clay soil.
It would be a further advancement in the art to provide a multi-shaft auger system which uses less energy when penetrating clay soils.
It would be a still further advancement in the art to provide a multi-shaft auger system which prevents the formation of clay soil cylindrical plugs below the support structures of the auger system.